1. Field of the Invention
The present invention relates to a distortion compensation quadrature modulator and a radio transmitter, and more particularly, to a distortion compensation quadrature modulator and a radio transmitter that output a highly accurate quadrature modulated wave by means of an analog quadrature modulator.
2. Description of Related Art
For a digital modulation scheme used in a mobile communication system, such as W-CDMA and PDC, for which the usable band of radio frequencies is limited, it is necessary to use a modulator with a higher degree of accuracy than in an analog modulation scheme, such as AM and FM. Although it is negligible for a quadrature modulator comprising a digital circuit, it is crucial to compensate for three kinds of linear distortions as specified below for a quadrature modulator comprising an analog element.
Firstly, for output signals I(t) and Q(t) from a digital-to-analog converter used as inputs to a quadrature modulator, it is difficult to completely adjust an offset of a DC component generated with respect to an original null-balance modulating signal. In addition, the DC offset shifts due to a change in temperature and a change with time, and the shift of the DC offset allows a carrier leak component to be superimposed on the modulated wave.
Secondly, an average amplitude ratio of the analog modulating signals I(t) and Q(t) shifts from the original ratio of “1”, which allows a distortion component to be superimposed on an image frequency region.
Thirdly, although a quadrature modulator needs a π/2 phase shifter, it is difficult to manufacture the π/2 phase shifter with precision, and a deviation in orthogonality causes a distortion component to be superimposed on an image frequency region.
In particular, it has been becoming increasingly important to solve problems resulting from the analog element in a direct conversion method that is expected to achieve a reduction of a circuit size by converting a baseband signal directly to an RF frequency.
These problems are solved by the technique disclosed in Hiroshi SUZUKI et al, “AFFINE-henkan senkeihizumi hoshou—idou-musen tsuusin niokeru touka o fukumu senkeisingoudensou heno tekiyou”, IEICE Transactions B-II, January 1992, Vol. J75-B-II, No. 1, pp. 1-9. FIG. 15 shows a schematic configuration according to this technique.
Referring to FIG. 15, numeral 120 denotes a distortion compensation circuit, numeral 3 denotes a quadrature modulator, numeral 150 denotes a wave detector that detects an RF transmission signal, numeral 160 denotes an LPF that equalizes detection outputs, and numeral 170 denotes a control circuit that generates distortion compensation coefficients and a test pattern. An affine transformer 121 has a configuration as shown in FIG. 2, and functions to correct a DC offset, an IQ gain difference, and a deviation in orthogonality present in the quadrature modulator by providing a0, b0, α, and θ. A concrete control method is as follows. That is, a test pattern signal is outputted by switching an SW2 and an SW3 to the control circuit 170, and DC offsets a′ and b′ are first found and set by observing an output from the LPF 160. Then, a test pattern signal is transmitted by switching an SW0 and an SW1 to the control circuit 170 while switching the SW2 and SW3 to the affine transformer 121, and α and θ are found by observing an output level of the LPF 160.
According to the technique in the related art as described above, however, respective distortion compensation coefficients are derived by transmitting a test pattern signal on the assumption that the coefficients are set at the shipment from the factory or set again by suspending the operation. Hence, respective distortion compensation coefficients cannot bederivedwhile the modulating signals I(t) and Q(t) essential for enabling communications are transmitted. The technique in the related art, therefore, is not applicable to a case where transmission cannot be suspended in a reliable manner to derive distortion compensation coefficients under operating conditions, for example, a base station that transmits signals at random timing. This technique therefore has a problem that it fails to address a change in temperature and a change with time.
The technique in the related art may be improved in such a manner that modulating signals are switched to a quadrature modulator in another system to secure a modulation output while a test pattern signal is transmitted. However, it is difficult to switch systems in complete absence of interruption with waves, and this alternative is not practical in terms of the cost and due to the incapability of addressing changes.